Toy Box Terror. Yule Jack can fit into spaces as small as 1 foot wide.

Chaotic Glee. Any creature that starts its turn within 10 feet of Yule Jack that can hear
his infectious giggling must succeed on a DC 15 Wisdom saving throw or become
incapacitated until the start of its next turn. On a successful saving throw, the creature
is immune to Yule Jack’s chaotic glee for 24 hours

Multiattack. Yule Jack makes two attacks: one with his swipe and one with his bite. 

Swipe. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 11 (2d8 +2) bludgeoning damage. If the target is a Medium or smaller creature, it is grappled (escape DC 15). Yule Jack can grapple two creatures at a time.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 11 (2d8 +2) piercing damage.

Swallow. Yule Jack makes one bite attack against a Medium or smaller target it is
grappling. If the attack hits, the target is also swallowed, and the grapple ends.
While swallowed, the target is blinded and restrained, it has total cover against attacks
and other effects outside Yule Jack, and it takes 21(6d6) acid damage at the start of each of Yule Jack's turns. Yule Jack can have only one creature swallowed at a time.

If Yule Jack takes 20 damage or more on a single turn from the swallowed creature, Yule Jack must succeed on a DC 15 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of Yule Jack. 

If Yule Jack dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.

Gleeful Pounce (Recharge 5-6). Yule Jack launches himself forward, leaping up to 20
feet in a straight line. Each creature in his path must make a DC 15 Dexterity saving
throw or take 14 (4d6) bludgeoning damage and fall prone.